1. Field of the Invention
The invention relates to a generative process for producing shaped elastomeric bodies from crosslinkable silicone compositions by means of electromagnetic radiation, this process being characterized in that the shaped elastomeric body is built up step by step, by repeated precise placement of small portions of the crosslinkable silicone composition and crosslinking thereof by means of electromagnetic radiation.
2. Description of the Related Art
For the production of shaped elastomeric parts starting from crosslinkable silicone rubber compositions there are numerous processing methods available. Depending on the consistency and mechanism of crosslinking of the silicone rubber composition, shaped parts may be produced, for example, by injection molding, by compression molding, by extrusion, by calendering, casting, etc. The properties of the shaped silicone part formed (hardness, tensile strength, extensibility, color, etc.) are determined substantially by the physical makeup of the crosslinkable silicone rubber composition, and further by the processing conditions such as pressure and temperature. In other words, these processes yield high unit numbers of shaped silicone parts which are typically largely isotropic in terms of the mechanical and optical properties.
The existing processes, however, are increasingly coming to their limits when the requirement is for shaped silicone parts of relatively complex geometry, of different material composition and/or with variable profiles of properties. The production of, for example, injection molds suitable for the particular purpose is becoming increasingly more costly and inconvenient or is fundamentally impossible. Profiles of requirements of this kind exist, for example, in the field of exo- and endoprostheses and especially epitheses (e.g., artificial outer ears, whose softer and harder areas (skin, cartilage) merge continuously into one another). In addition, very complex structures, such as those known from bionics, cannot be realized by means of the conventional processing techniques. The general trend toward individualization and individual adaptation of commodity articles, moreover, entails smaller unit numbers, thereby robbing conventional processes of their efficiency. The same is true of the production of prototypes.
One process for producing shaped parts that is gaining increasingly in importance is that of generative fabrication (Additive Manufacturing; 3D printing processes), which is an umbrella term for numerous different technologies whose common feature is an automated additive buildup of layers of the shaped part (A. Gebhardt, Generative Fertigungsverfahren, Carl Hanser Verlag, Munich 2013). A precondition of all generative production methods is the representation of the geometry and possibly of other properties (color, material composition) of the desired shaped body in the form of a digital 3D data set, which can be understood as a virtual model of the shaped body. This modeling is accomplished preferably by means of diverse 3D CAD construction methods (computer-aided design). The input data for the production of a 3D CAD model may also be 3D measurement data, as resulting, for example, from CT measurements (Computer Tomography) or MRT measurements (Magnetic Resonance Tomography). The 3D CAD data set must subsequently be supplemented by data specific to material, process, and fabricating unit, and this is accomplished by handing over the data set via an interface in a suitable format (e.g., STL, CLI/SLC, PLY, VRML, AMF format) to a piece of additive manufacturing software. From the geometric information, lastly, this software generates virtual individual layers (slices), and account is taken of the optimum orientation of the component in the construction space, support structures, etc. The complete data set then allows direct driving of the machine (3D printer) used for the generative fabrication.
The software sequence is as follows:    1. Construction of the component in CAD format    2. Export into the STL data format    3. Division of the 3D model into slices parallel to the printing plane and generation of the G-code    4. Transfer of the G-code to the printer control
Generative fabrication processes are available for numerous materials and also combinations thereof (e.g., metals, plastics, ceramics, glasses).
One of these technologies is the stereolithography process, described for example in WO 93/08506 A1 and WO 95/25003 A1, in which a radiation-curing plastics composition of low viscosity is applied in a thin layer to a support plate and is cured locally by computer-controlled irradiation of selected regions of the x, y plane (laser or radiation source with mask). After the next thin layer has been applied (typically by gauged lowering/immersion of the already crosslinked layer located on the support plate into a bath consisting of the noncrosslinked liquid plastics composition), this operation is repeated, the shaped body being built up layer by layer in the z-direction. Application of each next noncrosslinked layer with defined layer thickness can be accomplished, for example, by a doctor blade or a slot die. Lastly, the noncrosslinked plastics composition is removed from the resulting shaped body, optionally with assistance of a solvent and ultrasound. A process of this kind is also described in DE 199 50 284 A1, where one of the possibilities for the composition which cures by radical polymerization in visible light is an initiator-containing silicone resin composition, more particularly of the Ormocer® type. The silicone resin composition may comprise fillers and accordingly may also be of pastelike consistency. In that case, the individual layers can be applied by a roller system. A disadvantage is that the application of high-viscosity compositions by means of a roller system requires sheetlike substructures of high stability, a requirement which is generally not met by flexibly resilient substructures. WO 96/30182 as well describes the production of a shaped body in the stereolithography process starting from an epoxy-functional silicone composition which comprises a cationic photoinitiator and which is cured using a UV laser. The epoxy-functional monomers and oligomers used yield hard shaped bodies having a glass transition temperature of >250° C. Disadvantages would have to include not only the toxicological objectionability of cationic photoinitiators (onium salts) and photosensitizers (fused aromatics) but also the high density of epoxy group functions required in the silicone. Furthermore, the stereolithography process requires plastics compositions of relatively low viscosity (typically <0.3 Pa·s) in order to ensure effective leveling of the newly applied layers, which are a few hundred μm in thickness. The production of flexibly resilient shaped bodies with good mechanical strength, however, requires that long-chain silicone polymers be used in combination with reinforcing fillers, and this automatically results in silicone rubber compositions of comparatively high viscosity (>100 Pa·s). It is desirable, moreover, to use the physiologically unobjectionable addition-crosslinkable silicone rubber compositions for producing the shaped bodies; these compositions crosslink by platinum-catalyzed addition reaction of siloxane crosslinkers containing SiH groups onto vinyl-functional polysiloxanes (hydrosilylation). The combination of the thermally initiated or UV-light-initiated addition crosslinking with the above-described stereolithography process is a problem, since even when the irradiated and thus activated region is strongly focused (by means of an IR or UV laser), the immediate vicinity as well is crosslinked at least incipiently through diffusion of the activated platinum catalyst, through thermal conductivity, and through the strong exothermic nature of the addition-crosslinking process; this at least incipient crosslinking results in the entire silicone rubber composition in the bath being unusable, and also in the shaped body having a sticky, poorly delimited surface.
The above problem also cannot be solved by the process, disclosed in DE 100 24 618 A1, for producing three-dimensional articles from heat-sensitive compositions (those mentioned include silicone rubber resins), which is distinguished by the fact that the shaped body is formed by focused guidance of a light beam (focal spot of an IR laser) in three dimensions within the bath volume of a thermally crosslinkable liquid plastics composition, which must necessarily be transparent. Furthermore, the production of a flexibly resilient shaped body is tied to the presence of appropriate support structures, which cannot be constructed simultaneously in this process. Refinements of this process are described in DE 101 11 422 A1 and DE 101 52 878 B4. DE 101 52 878 B4 describes the production of three-dimensional shaped bodies or structures on surfaces by site-selective solidification of an organo-functional silicone resin within a bath of this material by means of two-photon or multiple-photon polymerization (A. Ostendorf, B. N. Chichov, Photonics Spectra 2006, 10, 72-80). The liquid silicone resins used contain organofunctional groups which permit (radical) two-photon or multiple-photon polymerization (e.g., methacrylic groups), with the high radiation intensity required for multiple-photon operations being generated by focusing of an ultrashort-time-pulsed (N)IR laser beam (optionally after beam widening beforehand). The advantage is that the radiation intensity required for the multiple-photon operation is achieved only within the focus area, while the material in the surroundings undergoes only one-photon excitation, which is unable to initiate the polymerization step, in other words the curing. The process is especially suitable for producing precisely shaped, substrate-supported or self-supporting structures preferably in the μm range, consisting of highly crosslinked, hard (ORMOCER®-like) organo-silicates, but not flexibly resilient silicone elastomers.
Overall it may be stated that the production of shaped rubber-elastic silicone bodies by the stereolithography-like processes above is unsuitable on account of the inadequacies recited. Something the above-recited generative processes have in common is that the noncrosslinked material forming the shaped body is not applied selectively on regions of a working surface (or is present as a bath), but instead is also present in regions which do not become part of the shaped body. Only through the subsequent selective curing does a part of the composition applied or present become a constituent of the shaped part. One embodiment of the stereolithography process that differs from this involves the crosslinkable composition being placed selectively only at those locations which do become part of the shaped body. The subsequent crosslinking may then take place selectively (by means of a laser, for example) or nonselectively (by areal irradiation with a lamp, for example).
The site-selective application of the crosslinkable composition may be accomplished by means of extrusion, for example. DE 10 2012 204 494 A1 describes the production of a primary silicone contact material for wound management, such production being able to take place by the 3D printing process among others. The primary contact material possesses the form of a lattice or mesh, which is formed, in analogy to filament 3D printing, by meanderlike continuous extrusion of silicone rubber compositions through a nozzle and subsequent crosslinking.
The site-selective application of the crosslinkable composition is accomplished preferably by what are called ballistic processes, a feature of which is that the crosslinkable material is applied with the aid of a printing head in the form of individual droplets, discontinuously, at the desired location of the work plane (Jetting). DE 10 2011 012 412 A1 and DE 10 2011 012 480 A1 describe an apparatus and a process for the step-by-step production of 3D structures with a printing head arrangement comprising at least two, preferably 50 to 200 printing head nozzles, allowing the site-selective application where appropriate of a plurality of photocrosslinkable materials with different photosensitivities, with the photocrosslinkable materials being subsequently subjected to site-selective solidification through electromagnetic radiation, more particularly through two-photon or multiple-photon processes in the focal region of a laser. The application of the photocrosslinkable materials by means of inkjet printing imposes specific requirements on the viscosity of the photocrosslinkable materials. Thus the photocrosslinkable materials feature a viscosity of less than 200 mPa·s, more preferably less than 80 mPa·s, and most preferably less than 40 mPa·s. In order to achieve sufficient crosslinking of the applied material by means of two-photon or multiple-photon polymerization, photoinitiators tailored to the laser wavelength are required, as is a polymeric crosslinker component containing photocrosslinkable groups, where the photocrosslinkable groups belong to the class of the acrylates, methacrylates, acrylamides, methylacrylamides, urethane acrylates, urethane methacrylates, urea acrylates, and urea methacrylates. The process described, however, is not suitable for producing shaped parts consisting of silicone elastomers. First of all, the solubility of the photoinitiators, photosensitizers, coinitiators, etc. that are used is poor in the (nonpolar) silicone compositions, and this leads to instances of hazing, microphase separation, and inhomogeneities. The radical curing of silicones functionalized with the abovementioned photocrosslinkable groups suffers, as is known, from the problem of inhibition caused by oxygen, thereby considerably reducing the crosslinking rate and resulting in sticky surfaces. If this effect is counteracted by raising the density of, for example, acrylate functional groups, the resulting vulcanizates are brittle and inelastic. Lastly, the extremely high local photon density that is required for multiple-photon polymerization (owing in particular to the low density of photopolymerizable group functions), and which is generated by means of pulsed femtosecond lasers, sets off decomposition reactions (carbonization) within the silicone, leading to unacceptable discolorations and damage to material.
It can be observed overall that none of the apparatuses and processes conforming to the prior art are suitable for efficient and effective production of high-quality shaped parts made of silicone elastomers in a generative fabrication process.